chore: improve signal perf by using Set rather than array for reactions

.changeset/calm-clocks-raise.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: improve signal perf by using Set rather than array for reactions

packages/svelte/src/internal/client/reactivity/sources.js

@@ -139,10 +139,8 @@ function mark_reactions(signal, status) {
 	if (reactions === null) return;
 
 	var runes = is_runes();
-	var length = reactions.length;
 
-	for (var i = 0; i < length; i++) {
-		var reaction = reactions[i];
+	for (var reaction of reactions) {
 		var flags = reaction.f;
 
 		// Skip any effects that are already dirty

packages/svelte/src/internal/client/reactivity/types.d.ts

@@ -9,7 +9,7 @@ export interface Signal {
 
 export interface Value<V = unknown> extends Signal {
 	/** Signals that read from this signal */
-	reactions: null | Reaction[];
+	reactions: null | Set<Reaction>;
 	/** Equality function */
 	equals: Equals;
 	/** The latest value for this signal */

packages/svelte/src/internal/client/runtime.js

@@ -169,7 +169,7 @@ export function check_dirtiness(reaction) {
 
 			if ((flags & DISCONNECTED) !== 0) {
 				for (i = 0; i < dependencies.length; i++) {
-					(dependencies[i].reactions ??= []).push(reaction);
+					(dependencies[i].reactions ??= new Set()).add(reaction);
 				}
 
 				reaction.f ^= DISCONNECTED;
@@ -188,11 +188,11 @@ export function check_dirtiness(reaction) {
 
 				if (is_unowned) {
 					// TODO is there a more logical place to do this work?
-					if (!current_skip_reaction && !dependency?.reactions?.includes(reaction)) {
+					if (!current_skip_reaction && !dependency?.reactions?.has(reaction)) {
 						// If we are working with an unowned signal as part of an effect (due to !current_skip_reaction)
 						// and the version hasn't changed, we still need to check that this reaction
 						// if linked to the dependency source – otherwise future updates will not be caught.
-						(dependency.reactions ??= []).push(reaction);
+						(dependency.reactions ??= new Set()).add(reaction);
 					}
 				}
 			}
@@ -323,17 +323,7 @@ export function update_reaction(reaction) {
 
 			if (!current_skip_reaction) {
 				for (i = skipped_deps; i < deps.length; i++) {
-					dependency = deps[i];
-					var reactions = dependency.reactions;
-
-					if (reactions === null) {
-						dependency.reactions = [reaction];
-					} else if (
-						reactions[reactions.length - 1] !== reaction &&
-						!reactions.includes(reaction)
-					) {
-						reactions.push(reaction);
-					}
+					(deps[i].reactions ??= new Set()).add(reaction);
 				}
 			}
 		} else if (deps !== null && skipped_deps < deps.length) {
@@ -358,24 +348,16 @@ export function update_reaction(reaction) {
  * @returns {void}
  */
 function remove_reaction(signal, dependency) {
-	const reactions = dependency.reactions;
-	let reactions_length = 0;
+	let reactions = dependency.reactions;
 	if (reactions !== null) {
-		reactions_length = reactions.length - 1;
-		const index = reactions.indexOf(signal);
-		if (index !== -1) {
-			if (reactions_length === 0) {
-				dependency.reactions = null;
-			} else {
-				// Swap with last element and then remove.
-				reactions[index] = reactions[reactions_length];
-				reactions.pop();
-			}
+		reactions.delete(signal);
+		if (reactions.size === 0) {
+			reactions = dependency.reactions = null;
 		}
 	}
 	// If the derived has no reactions, then we can disconnect it from the graph,
 	// allowing it to either reconnect in the future, or be GC'd by the VM.
-	if (reactions_length === 0 && (dependency.f & DERIVED) !== 0) {
+	if (reactions === null && (dependency.f & DERIVED) !== 0) {
 		set_signal_status(dependency, MAYBE_DIRTY);
 		// If we are working with a derived that is owned by an effect, then mark it as being
 		// disconnected.

packages/svelte/tests/signals/test.ts

@@ -236,13 +236,13 @@ describe('signals', () => {
 		return () => {
 			flushSync(() => set(count, 1));
 			// Ensure we're not leaking consumers
-			assert.deepEqual(count.reactions?.length, 1);
+			assert.deepEqual(count.reactions?.size, 1);
 			flushSync(() => set(count, 2));
 			// Ensure we're not leaking consumers
-			assert.deepEqual(count.reactions?.length, 1);
+			assert.deepEqual(count.reactions?.size, 1);
 			flushSync(() => set(count, 3));
 			// Ensure we're not leaking consumers
-			assert.deepEqual(count.reactions?.length, 1);
+			assert.deepEqual(count.reactions?.size, 1);
 			assert.deepEqual(log, [0, 1, 2, 3]);
 		};
 	});
@@ -304,8 +304,8 @@ describe('signals', () => {
 			flushSync(() => set(count, 4));
 			flushSync(() => set(count, 0));
 			// Ensure we're not leaking consumers
-			assert.deepEqual(count.reactions?.length, 1);
-			assert.deepEqual(calc.reactions?.length, 1);
+			assert.deepEqual(count.reactions?.size, 1);
+			assert.deepEqual(calc.reactions?.size, 1);
 			assert.deepEqual(log, [0, 2, 'limit', 0]);
 			destroy();
 			// Ensure we're not leaking consumers
@@ -484,7 +484,7 @@ describe('signals', () => {
 		return () => {
 			flushSync();
 			assert.equal(a?.deps?.length, 1);
-			assert.equal(state?.reactions?.length, 1);
+			assert.equal(state?.reactions?.size, 1);
 			destroy();
 			assert.equal(a?.deps?.length, 1);
 			assert.equal(state?.reactions, null);
@@ -658,12 +658,12 @@ describe('signals', () => {
 			});
 
 			assert.equal($.get(d), 0);
-			assert.equal(count.reactions?.length, 1);
+			assert.equal(count.reactions?.size, 1);
 			assert.equal(d.deps?.length, 1);
 
 			set(count, 1);
 			assert.equal($.get(d), 2);
-			assert.equal(count.reactions?.length, 1);
+			assert.equal(count.reactions?.size, 1);
 			assert.equal(d.deps?.length, 1);
 
 			destroy();